How to set user task extension value as variable

I am getting an error when placing a variable in the formName value area but no issue if the value is hardcoded. Any ideas how I can set this with a variable i pass from a previous bpmn step?

I am getting the following error when the system is checking the variable name…

Caused by: org.camunda.bpm.engine.ProcessEngineException: Exception while invoking TaskListener: Not enough variable values available to expand ‘stepaformname’

Thanks in advance :wink: